The invention relates to a current transformer comprising a magnetic circuit connected to a primary conductor, and a secondary winding designed to supply a secondary current to actuate a trip relay.
Current transformers of known types transform a primary current of strong intensity flowing in a conductor of a power system to be protected into a secondary current supplying circuitry and a trip relay. These transformers generally operate with alternating currents. In certain trip devices, the alternating secondary current is also used to supply a measurement signal to protection circuits.
For direct current power systems, transformers of known types do not enable a measurement signal or sufficient energy to supply circuitry or a trip relay to be efficiently supplied. In this case, an auxiliary voltage supply and an additional current measurement device are necessary. However, a large short-circuit may occur near to a circuit breaker comprising an auxiliary power supply. The power system voltage can then drop and cause a decrease of the voltage supplied by the auxiliary power supply. Such a decrease can give rise to tripping faults of the trip devices and be insufficient to supply the trip relay to open the circuit breaker.
Transformers can supply a secondary current when the variation of the primary current is large, even when said primary current has a DC polarity. The secondary current depends on the variation of the primary current. It can be used to actuate a trip relay of a circuit breaker if said variation is large. However, such a trip on a short-circuit only occurs above a high value of the derivative of the primary current.